The present invention relates to a video camera system whose lens assemblies are interchangeable.
Conventionally, a so-called hill-climbing method is known as the method of an automatic focusing device used in video apparatuses such as video cameras. The method performs focusing by extracting a high-frequency component from an image sensing signal obtained by an image sensing device such as a CCD and driving a taking lens such that the mountain-like characteristic curve of this high-frequency component is a maximum.
This automatic focusing method requires neither emission/reception of infrared rays nor special focusing optical members for detecting the movement of an image which changes in accordance with the state of a focus. The method also has an advantage in that an object can be accurately focused regardless of whether the distance to the object is long or short.
An example in which an automatic focusing method of the above sort is applied to an interchangeable lens video camera will be described below with reference to FIG. 15.
FIG. 15 is a block diagram showing an interchangeable lens video camera system as one prior art.
In FIG. 15, an automatic focusing system comprises a lens assembly 500 and a camera main body 550. Focusing is performed by driving a focus lens 501 in the direction of an optical axis by a lens driving motor 511. An image of light transmitting through this lens is formed on the image sensing surface of an image sensing device 502 and changed into an electrical signal by photoelectric conversion. This electrical signal is output as a video signal. The video signal is sampled-and-held and amplified to a predetermined level by a CDS/AGC (Correlated Double Sampling/Auto Gain Control) circuit 503, and converted into digital video data by an A/D (Analog/Digital) converter 504. The data is input to a process circuit (not shown) of the camera and converted into a standard television signal. The data is also input to a bandpass filter (to be referred to as BPF hereinafter) 505.
The BPF 505 extracts a high-frequency component from the video signal. A gate circuit 506 extracts only a signal corresponding to a portion set in an in-focus designated area in an image sensing surface. A peak hold circuit 507 holds peak values at intervals synchronized with integral multiples of a vertical sync signal, generating an AF (AutoFocus) evaluation value.
An AF microcomputer 508 of the camera main body 550 fetches this AF evaluation value and determines the driving velocity of a focus motor 511 in accordance with an in-focus degree and the driving direction of the motor along which the AF evaluation value increases. The AF microcomputer 508 transmits the driving velocity and the driving direction of the focus motor 511 to a microcomputer 509 of the lens assembly 500.
In accordance with the designations from the AF microcomputer 508 of the camera main body 550, the microcomputer 509 operates the focus motor 511 via a motor driver 510 to drive the focus lens 501 in the optical axis direction, thereby performing focusing.
In the above prior art, however, the camera main body has the function of controlling automatic focusing in order to allow an interchange of lenses. Therefore, if, for example, the response characteristics of automatic focusing are so determined as to be optimum for a specific lens, the characteristics may not be optimum for other lenses, resulting in a low versatility.
A problem arising when an interchangeable lens is a zoom lens will be described below with reference to FIG. 16.
FIG. 16 is a block diagram of an interchangeable zoom lens video camera system as another prior art.
In a conventional variable power lens assembly, a variable power lens 21 and a compensating lens 22 are mechanically connected by a cam. When a zooming operation is manually or electrically performed, the variable power lens 21 and the compensating lens 22 integrally move.
These variable power lens 21 and compensating lens 22 are called zoom lenses. In this lens system, a lens (front lens) 1 which is closest to an object when the image is taken is a focus lens. The focus lens 1 moves in the direction of an optical axis to perform focusing.
An image of light transmitting through these lenses is formed on the image sensing surface of an image sensing device 3, photoelectrically converted into an electrical signal, and output as a video signal. This video signal is sampled-and-held (correlated double sampling) by a CDS/AGC circuit 4, amplified to a predetermined level by AGC (Auto Gain Control), and converted into digital video data by an A/D converter 5. The digital video data is input to a subsequent camera process circuit (not shown) and converted into a standard television signal. The data is also input to an AF signal processing circuit 6.
The AF signal processing circuit 6 extracts a high-frequency component which changes in accordance with the focus state from the video signal. A microcomputer 7 for controlling the system fetches this high-frequency component as an AF evaluation value.
The microcomputer 7 determines the driving velocity of a focus motor in accordance with the in-focus degree and the driving direction of the motor along which the AF evaluation value increases. The microcomputer 7 sends the velocity and the direction of the focus motor to a focus motor driver 9 of a lens assembly 12 and drives the focus lens 1 via a focus motor 10.
The microcomputer 7 also reads the state of a zoom switch 8 and, in accordance with the operation state of the zoom switch 8, determines the driving directions and the driving velocities of the zoom lenses 21 and 22. The microcomputer 7 transmits these driving directions and driving velocities to a zoom motor driver 11 of the lens assembly 12 and drives the zoom lenses 21 and 22 via a zoom motor 12.
A camera main body 13 can be separated from the lens assembly 12 and connected to another lens assembly. This widens the range of shooting.
In recent integrated cameras for consumers having the above structure, the cam for mechanically connecting the compensating lens with the variable power lens is no longer used in order to miniaturize a camera and enable shooting at a close distance such as when an object is almost at the front surface of the lens. In these cameras, the locus of movement of the compensating lens is previously stored as lens cam data in a microcomputer, and the compensating lens is driven in accordance with this lens cam data. Also, a focusing operation is performed by using this compensating lens. Lenses of this type, i.e., so-called inner focus type (rear focus type) lenses have become most popular.
According to the technical concept of the above prior art, however, all control operations are done in the camera main body, and the lens assembly is driven in accordance with control signals supplied from the camera main body. Therefore, to use an inner focus type lens as an interchangeable lens assembly, the camera main body must have the data of the locus of movement of the focus lens, i.e., the lens cam data, for maintaining the in-focus state by compensating for a change in the focal plane caused by a zooming operation.
This, however, imposes on the camera main body the serious burden of having the lens cam data which differs from one lens assembly to another. Accordingly, the method becomes unrealistic as the number of interchangeable lenses increases.